Chemical Hydrophobation
Chemical modification process applied at the paper machine size press to impart liquid penetration resistance using reactive hydrophobic synthetic compounds. Paperboard mills run surface sizing AKD ASA formulations to control aqueous liquid absorption and improve offset printability. Alkyl ketene dimer and alkenyl succinic anhydride react directly with cellulose hydroxyl groups, anchoring long hydrophobic hydrocarbon tails outward from the fiber surface.
ASA reacts rapidly within seconds during drying, whereas AKD requires post-dryer curing time to achieve full water repellency. The surface treatment applies to the paper web exterior, leaving internal fiber matrices largely unreacted unless combined with wet-end sizing.
Anchoring Kinetics
Size press applications meter starch suspensions containing emulsified reactive sizing agents onto partially dried paper webs. Heat in the dryer section melts the wax-like sizing particles, spreading active molecules across fiber intersections before covalent esterification occurs. The hydrophobic barrier elevates liquid contact angles, preventing ink spreading, dampening solution penetration and feathering during high-speed offset printing.
ASA requires precise emulsification with cationic starch immediately prior to application due to rapid hydrolysis in water. AKD provides stable emulsion shelf life but exhibits migration tendencies if excess unreacted wax remains on the sheet surface.
Reversibility Boundary
Excessive sizing chemical application causes slippery paper surfaces, lowering friction coefficients and causing stacking instability in sheet cutters. High temperatures or extreme alkaline repulping break ester bonds, reversing hydrophobicity during paper recycling processes.
